1. Field of the Invention
This invention relates to nonwoven fabrics for filters adapted for separating and removing dusts in the air, etc., and more particularly to nonwoven fabrics for filters excellent in retention volume of trapped matters such as dusts and carbon particles and capable of maintaining high cleaning efficiency for a long term, a process for manufacturing such nonwoven fabrics, and filters using such nonwoven fabrics.
2. Description of the Related Art
Heretofore, there has been customarily used a method for allowing air containing dusts, etc. to pass through filters for removal of the dusts, etc., as a method for providing clean air. Performance required for filters differs depending on what the filters are used for. In view of this, a variety of materials such as paper and fibers are used as materials for filters. In case that filters are used for air-intake engines of automotive vehicles, the ability of keeping high cleaning efficiency as well as attaining high cleaning efficiency is required.
Recently, nonwoven fabrics of density-gradient type have been proposed as filters having improved maintainability of cleaning efficiency by removing and holding dusts. Nonwoven fabric of density gradient type is nonwoven fabric in which fibrous layers each having a different fibrous density are stacked one over the other in such a manner that the density is varied stepwise in the stacked direction of the fibrous layers. Each fibrous layer is composed of fibers having different fiber diameters. More specifically, the nonwoven fabric of density gradient type is constructed in such a manner that the density of each fibrous layer is increased stepwise in the stacked direction of the fibrous layers from one surface of the nonwoven fabric to the opposite surface thereof by changing the mixing ratio of the fibers composing the fibrous layer with respect to each fibrous layer. In such nonwoven fabric, fibers are bonded to each other by adhesion with a variety of binders, or by adding adhesive fibers and by fusing part of the adhesive fibers so that the fused adhesive fibers are adhered to the other fibers.
Even the abovementioned nonwoven fabric of density-gradient type cannot provide sufficient cleaning efficiency of removing micro particles such as carbon particles. Accordingly, there has been proposed a technique of improving cleaning efficiency by increasing the density of the fibrous layers. Some of the exemplified techniques of increasing the fibrous density are such that the size of the pore (dimension of the clearance) defined by each adjacent fibers is reduced by using fibers having a very small diameter and that the pore size is reduced by impregnating resin in the fibers. In case where such nonwoven fabric is used as a filter, the filter is highly likely to result in clogging of the clearances (pores) in the fibrous layers even if the cleaning efficiency of removing micro particles such as carbon particles is secured. As a result, the life of such a filter is shortened. Furthermore, increasing the density of the fibrous layer may lower retention volume of the filter for trapped dusts and carbon particles, which is resulted in lowering life span of the filter.
In an attempt to solve the above problems, a variety of nonwoven fabrics for filters have been proposed recently. For example, Japanese Unexamined Patent Publication No. HEI 9-192427 discloses the following technique. Specifically, at least two fibrous layers each composed of thermo-adhesive hydrophobic fibers and non-thermal-adhesive hydrophobic fibers are stacked one over the other such that the fibrous layer on an air outlet side has an average fineness smaller than the fibrous layer on an air inlet side. The thus prepared nonwoven fabric is washed with water to reduce the content of oil agent attached to the fibers. Thereby, produced is the nonwoven fabric having improved cleaning performance of trapping sand dusts while suppressing clogging in the fibrous layers by the accumulated dusts.
Japanese Unexamined Patent Publication No. SHO 50-31476 discloses the nonwoven fabric produced by preparing a web containing at least 30% of composite fiber constructed such that fibers made of polymer capable of forming two kinds of fibers with a melting point difference at 20° C. or more are arrayed in a row, subjecting the web to a needle punch processing under a condition where the active barb number ranges from 40 to 150, and subjecting the thus processed web to a thermal treatment at a temperature not lower than a melting point of the low-melting-point component of the composite fiber and lower than the melting point of the high-melting-point component thereof.
The filters for engines of automotive vehicles require high retention volume of dusts, etc., as well as cleaning efficiency of the dusts, etc. considering the demand that the filters be used for a long term. Further, in view of the recent demand for cost reduction, demand for production of nonwoven fabrics capable of providing high cleaning efficiency and high retention volume of dusts, etc. with a less number of fibrous layers and excellent in formability and integral formability has increased.